A-Level生物 基因表达 蛋白质合成

A-Level生物 基因表达 蛋白质合成

1. 引言：从基因到蛋白质 Introduction: From Gene to Protein

Gene expression is the process by which the information encoded in a gene is used to direct the synthesis of a functional gene product, typically a protein. This is the central dogma of molecular biology: DNA makes RNA makes protein. The flow of genetic information is unidirectional under normal circumstances, moving from DNA (transcription) to mRNA (translation) to polypeptide. Understanding gene expression is fundamental to A-Level Biology because it explains how genotype determines phenotype and how cells differentiate to perform specialised functions despite carrying identical DNA. The process occurs in two major stages: transcription in the nucleus, and translation in the cytoplasm at ribosomes.

基因表达是指基因中编码的信息被用来指导合成功功能性基因产物（通常是蛋白质）的过程。这就是分子生物学的中心法则：DNA制造RNA，RNA制造蛋白质。在正常情况下，遗传信息的流动是单向的，从DNA（转录）到mRNA（翻译）再到多肽。理解基因表达对A-Level生物至关重要，因为它解释了基因型如何决定表型，以及细胞如何在携带相同DNA的情况下分化以执行特殊功能。该过程分为两个主要阶段：细胞核中的转录和细胞质中核糖体上的翻译。

2. 转录：DNA到mRNA Transcription: DNA to mRNA

Transcription is the first stage of gene expression and takes place in the nucleus of eukaryotic cells. The enzyme RNA polymerase binds to a specific region of DNA called the promoter, which is located just upstream of the gene. The DNA double helix unwinds and unzips, exposing the template strand (also called the antisense strand). RNA polymerase moves along the template strand in the 3′ to 5′ direction, synthesising a complementary mRNA molecule in the 5′ to 3′ direction by adding free RNA nucleotides according to the base-pairing rules: adenine pairs with uracil (instead of thymine), cytosine pairs with guanine, guanine pairs with cytosine, and thymine pairs with adenine. The coding strand (sense strand) has the same sequence as the mRNA, except thymine is replaced by uracil. Transcription continues until RNA polymerase reaches a terminator sequence, at which point the newly synthesised pre-mRNA molecule detaches and the DNA rewinds into its double-helix structure.

转录是基因表达的第一阶段，发生在真核细胞的细胞核中。RNA聚合酶结合到DNA的特定区域，称为启动子，位于基因的上游。DNA双螺旋解旋并解链，暴露出模板链（也称为反义链）。RNA聚合酶沿模板链的3’到5’方向移动，通过碱基配对规则添加游离的RNA核苷酸，沿5’到3’方向合成互补的mRNA分子：腺嘌呤与尿嘧啶配对（而不是胸腺嘧啶），胞嘧啶与鸟嘌呤配对，鸟嘌呤与胞嘧啶配对，胸腺嘧啶与腺嘌呤配对。编码链（有义链）与mRNA具有相同的序列，只是胸腺嘧啶被尿嘧啶取代。转录持续进行，直到RNA聚合酶到达终止序列，此时新合成的pre-mRNA分子脱落，DNA重新缠绕回双螺旋结构。

3. RNA加工：剪接与修饰 RNA Processing: Splicing and Modification

In eukaryotic cells, the primary transcript (pre-mRNA) undergoes several processing steps before it can leave the nucleus and be translated. First, a modified guanine nucleotide cap (5′ cap) is added to the 5′ end, which protects the mRNA from degradation and helps ribosomes recognise the mRNA during translation. Second, a poly-A tail (a string of approximately 200 adenine nucleotides) is added to the 3′ end, which also protects the mRNA and facilitates its export from the nucleus. The most critical processing step is splicing: eukaryotic genes contain exons (coding regions) and introns (non-coding regions). The introns are removed and the exons are joined together by a complex called the spliceosome, which is composed of small nuclear ribonucleoproteins (snRNPs). This splicing must be precise ： even a single-nucleotide error at the splice site can cause a frameshift mutation in the final protein product. Alternative splicing allows a single gene to produce multiple different mRNA variants and therefore multiple different proteins, greatly increasing the diversity of the proteome.

在真核细胞中，初级转录物（pre-mRNA）在离开细胞核并被翻译之前需要经过几个加工步骤。首先，在5’端添加一个修饰的鸟嘌呤核苷酸帽（5’帽），它保护mRNA免遭降解，并帮助核糖体在翻译过程中识别mRNA。其次，在3’端添加poly-A尾（大约200个腺嘌呤核苷酸的序列），它同样保护mRNA并促进其从细胞核中输出。最关键的加工步骤是剪接：真核基因包含外显子（编码区域）和内含子（非编码区域）。内含子被去除，外显子由剪接体复合物连接在一起，剪接体由小核核糖核蛋白（snRNP）组成。这种剪接必须精确：：即使剪接位点上的单个核苷酸错误也可能导致最终蛋白质产物的移码突变。可变剪接允许一个基因产生多种不同的mRNA变体，从而产生多种不同的蛋白质，大大增加了蛋白质组的多样性。

4. 翻译：mRNA到蛋白质 Translation: mRNA to Protein

Translation is the process by which the genetic code carried by mRNA is decoded to produce a specific sequence of amino acids in a polypeptide chain. This process occurs at ribosomes in the cytoplasm. The ribosome reads the mRNA sequence in groups of three nucleotides called codons. Each codon specifies a particular amino acid, and the sequence of codons determines the primary structure of the protein. Transfer RNA (tRNA) molecules act as adaptors: each tRNA has a specific anticodon (three unpaired bases) that is complementary to an mRNA codon, and carries the corresponding amino acid at its 3′ end. The enzyme aminoacyl-tRNA synthetase attaches the correct amino acid to each tRNA ： this is a critical proofreading step because the accuracy of protein synthesis depends on correct amino acid loading. The ribosome has three binding sites for tRNA: the A site (aminoacyl), the P site (peptidyl), and the E site (exit). Translation proceeds through three phases: initiation, elongation, and termination.

翻译是解码mRNA携带的遗传密码以产生多肽链中特定氨基酸序列的过程。该过程在细胞质的核糖体上进行。核糖体以三个核苷酸为一组（称为密码子）读取mRNA序列。每个密码子指定一个特定的氨基酸，密码子的序列决定了蛋白质的一级结构。转运RNA（tRNA）分子充当接头：每个tRNA具有一个特定的反密码子（三个未配对的碱基），与mRNA密码子互补，并在其3’端携带相应的氨基酸。氨酰tRNA合成酶将正确的氨基酸连接到每个tRNA上：：这是一个关键的校对步骤，因为蛋白质合成的准确性取决于正确的氨基酸装载。核糖体具有三个tRNA结合位点：A位点（氨酰位）、P位点（肽基位）和E位点（出口位）。翻译通过三个阶段进行：起始、延伸和终止。

5. 翻译的起始与延伸 Initiation and Elongation of Translation

Initiation begins when the small ribosomal subunit binds to the 5′ cap of the mRNA and scans along until it finds the start codon, AUG, which codes for methionine. A special initiator tRNA carrying methionine binds to the AUG codon via its anticodon UAC, and the large ribosomal subunit then joins to form the complete translation complex. During elongation, a new aminoacyl-tRNA enters the A site, and if its anticodon matches the mRNA codon, the ribosome catalyses the formation of a peptide bond between the amino acid in the P site and the new amino acid in the A site. The ribosome then translocates ： moving three nucleotides along the mRNA ： which shifts the tRNA from the A site to the P site, and the uncharged tRNA from the P site to the E site, from which it exits. This process repeats, adding amino acids one by one to the growing polypeptide chain. Each peptide bond formation and translocation step requires energy from GTP hydrolysis. Elongation continues until a stop codon (UAA, UAG, or UGA) enters the A site.

起始阶段开始于小核糖体亚基结合到mRNA的5’帽上，并沿mRNA扫描直到找到起始密码子AUG，它编码甲硫氨酸。携带甲硫氨酸的特殊起始tRNA通过其反密码子UAC结合到AUG密码子上，然后大核糖体亚基加入形成完整的翻译复合物。在延伸过程中，一个新的氨酰tRNA进入A位点，如果其反密码子与mRNA密码子匹配，核糖体催化P位点上的氨基酸与A位点上的新氨基酸之间形成肽键。然后核糖体沿mRNA移动三个核苷酸：：将tRNA从A位点转移到P位点，将不带电的tRNA从P位点转移到E位点并排出。这个过程不断重复，逐个将氨基酸添加到不断增长的多肽链上。每次肽键形成和移位步骤都需要从GTP水解释放的能量。当一个终止密码子（UAA、UAG或UGA）进入A位点时，延伸停止。

6. 翻译终止与蛋白质折叠 Termination and Protein Folding

Stop codons are not recognised by any tRNA. Instead, when a stop codon enters the A site, a protein called release factor binds to it. This triggers the ribosome to add a water molecule to the end of the polypeptide chain instead of another amino acid, which causes the polypeptide to be released from the tRNA in the P site. The ribosomal subunits then dissociate and can be reused for another round of translation. The newly synthesised polypeptide chain must fold into its correct three-dimensional conformation to become functional. Folding is influenced by the amino acid sequence (primary structure) and assisted by chaperone proteins that prevent misfolding and aggregation. Some proteins require post-translational modifications such as phosphorylation, glycosylation, or cleavage before they become fully active. The protein is then directed to its cellular destination by signal sequences ： short amino acid motifs that act like postal codes, directing the protein to the endoplasmic reticulum, Golgi apparatus, mitochondria, or other organelles, or tagging it for secretion from the cell.

终止密码子不被任何tRNA识别。相反，当终止密码子进入A位点时，一种称为释放因子的蛋白质结合到上面。这触发核糖体在多肽链末端添加一个水分子而不是另一个氨基酸，导致多肽从P位点的tRNA上释放出来。核糖体亚基随后解离，可以重新用于另一轮翻译。新合成的多肽链必须折叠成其正确的三维构象才能发挥功能。折叠受氨基酸序列（一级结构）的影响，并由伴侣蛋白辅助防止错误折叠和聚集。一些蛋白质在完全活跃之前需要翻译后修饰，如磷酸化、糖基化或切割。然后蛋白质通过信号序列被引导到其细胞目的地：：短的氨基酸基序像邮政编码一样，将蛋白质引导到内质网、高尔基体、线粒体或其他细胞器，或标记其从细胞中分泌。

7. 基因表达的调控 Regulation of Gene Expression

Not all genes are expressed in all cells at all times. Gene expression is tightly regulated to ensure that the right proteins are produced in the right cells at the right time and in the right amounts. In eukaryotes, regulation can occur at multiple levels: transcriptional regulation involves transcription factors ： proteins that bind to specific DNA sequences (enhancers and silencers) near the promoter to activate or repress transcription. Epigenetic modifications, such as DNA methylation and histone acetylation, alter chromatin structure to make genes more or less accessible to the transcription machinery. Post-transcriptional regulation includes alternative splicing and mRNA degradation rates. Translational regulation controls the rate at which mRNA is translated into protein. The lac operon in E. coli is a classic prokaryotic example studied at A-Level, demonstrating how gene expression can be switched on or off in response to environmental conditions (presence or absence of lactose).

并非所有基因在所有细胞中随时表达。基因表达受到严格调控，以确保正确的蛋白质在正确的细胞、正确的时间以正确的数量产生。在真核生物中，调控可以发生在多个层面：转录调控涉及转录因子：：结合到启动子附近特定DNA序列（增强子和沉默子）以激活或抑制转录的蛋白质。表观遗传修饰，如DNA甲基化和组蛋白乙酰化，改变染色质结构，使基因对转录机器更容易或更难以接近。转录后调控包括可变剪接和mRNA降解速率。翻译调控控制mRNA翻译成蛋白质的速率。大肠杆菌中的乳糖操纵子是A-Level学习的经典原核生物例子，展示了基因表达如何根据环境条件（乳糖的存在或缺失）开启或关闭。

8. 考试技巧 Exam Tips

When answering A-Level exam questions on gene expression, remember to distinguish clearly between transcription and translation ： do not confuse the two. Be precise about the roles of specific enzymes: RNA polymerase for transcription, aminoacyl-tRNA synthetase for tRNA charging, and peptidyl transferase (a ribozyme activity of the large ribosomal subunit rRNA) for peptide bond formation. Use correct terminology: template strand (not “coding strand”) for the DNA strand that is transcribed; anticodon for tRNA; codon for mRNA. In questions about mutations, explain how a change in the DNA sequence can affect the primary structure of the protein, and hence its tertiary structure and function. For splicing questions, mention that introns are removed and exons are joined, and that alternative splicing produces multiple proteins from one gene. Draw clear, labelled diagrams showing the ribosome with A, P, and E sites, and the direction of mRNA movement through the ribosome (5′ to 3′).

在回答A-Level考试中关于基因表达的问题时，记住清楚区分转录和翻译：：不要混淆两者。要准确说明特定酶的作用：RNA聚合酶负责转录，氨酰tRNA合成酶负责tRNA装载，肽基转移酶（大核糖体亚基rRNA的核酶活性）负责肽键形成。使用正确的术语：模板链（而非”编码链”）是被转录的DNA链；反密码子用于tRNA；密码子用于mRNA。在关于突变的问题中，解释DNA序列的变化如何影响蛋白质的一级结构，进而影响其三级结构和功能。对于剪接问题，提到内含子被去除而外显子被连接，可变剪接从一个基因产生多种蛋白质。画出清晰、带标签的图表，显示带有A、P和E位点的核糖体，以及mRNA通过核糖体的方向（5’到3’）。